Four stroke diesel cycle internal combustion engines are well known. One of ordinary skill in the art will readily recognize that such engines operate through four distinct strokes of a piston reciprocating within a cylinder. In an intake stroke, the piston descends within the cylinder while an intake valve is open. Air is thereby able to enter the cylinder through the open intake valve. In a subsequent compression stroke, the piston reverses direction while the intake valve and an exhaust valve are closed, thereby compressing the air. This is followed by a combustion or power stroke wherein the fuel is directly injected into the compressed air and thereby ignited, with the resulting force pushing the piston again in the descending direction while both valves are closed. Finally, the piston reverses direction with the exhaust valve open, thereby pushing the combustion gases out of the cylinder.
Various types of valve timing schemes have been developed, for example, U.S. Pat. Nos. 4,535,732, 5,031,583, 5,280,770, 5,469,818, 7,055,472, 7,069,887, 7,255,075 and 7,347,171. Many of these patents use hydraulics to hold the valve mechanisms in place or use cam phasors to shift the phase of the cam relative to the crankshaft.
However, the optimized valve events for some operating conditions are not necessarily optimized over the entire operating range. In particular, valve strategies that have been investigated more recently, including Miller cycle strategies, can provide very good performance over a range of conditions, but some conditions, and in particular low to medium load, suffer from poor airflow.